Pallets are widely used in transporting and storing a wide variety of materials, documents, foods, component parts, in-process inventory, and articles of manufacture. Such products are typically loaded onto a pallet for ease of transportation and storage because the pallet can be easily moved with a forklift truck to move the items thereon to a desired location. For instance, goods of all types are commonly loaded onto a pallet for transportation by truck, rail, or ship and then stored on the pallet for subsequent use in a warehouse. Component parts and in-process inventory is also frequently transported within a factory or between factories for subsequent use in making completed products. For instance, in tire manufacturing operations, green tires (uncured tires) are initially built on tire building machines and subsequently delivered to cure presses for vulcanization into cured tires. In some cases, the green tires are stored on the pallets or buggies for some period of time before being cured as a function of in-process inventory flow within the tire plant.
Conventional pallets typically include a top plate, a bottom plate, and stringers which connect the top plate to the bottom plate and provide a desired spacing which is typically between 3.5 inches and 10 inches between the top plate and the bottom plate. For instance, the spacing between the top plate and the bottom plate will typically be between 3.5 inches in the case of wooden pallets. Pallets can be designed for two-way entry or four-way entry to facilitate transportation with forklift trucks. Pallets which are designed for general purpose applications typically have an essentially flat top plate which can consist of a single flat surface, such as a flat piece of plywood, or it can consist of a series of boards which are affixed to the stringers. Such pallets can be made of wood, plastic or metal. However, in some cases the pallet will be designed for transporting and/or storing a specific item. For instance, in the case of green tires the pallet may include a top plate having one or more tapered or venturi shaped cavities (holders) which are specifically adapted for receiving the green tires. The holders will be made with a material which has a surface which allows for ease of release of the green tire there from. In other words, the green tire should not have a propensity to adhere or stick to the holder. Such tire pallets typically include at least one such holder and can include many holders for receiving the green tires.
In state of the art facilities, automatic stacking and retrieval systems (ASRS) are frequently utilized to efficiently store pallets which are loaded with desired items. In such systems, the pallets are loaded onto simple pallet racks, drive-in/drive-thru racks, telescoping frame racks, gravity pallet flow racks, or pallet push back racks. In the case of simple pallet racks the pallets are loaded onto the rack in a configuration which is only one pallet deep. Simple pallet rack systems offer the advantage of being low cost and require low maintenance, but do not provide efficient utilization of warehouse space. This is because pathways need to be provided to allow access to at least one side of each rack space on which pallets are stored. Telescoping frame racks, gravity pallet flow racks, and pallet push back racks provide for much more efficient utilization of floor space. However, telescoping frame racks take more vertical space, are generally limited to being up to four pallets deep, are more costly, and require more maintenance due to their array of moving parts. Gravity pallet flow racks require access to both sides of each row in which pallets are stored (one side for insertion of the pallet, and the other side for pallet removal). Accordingly, pallet push back racks make more efficient use of warehouse space because these racks can store more pallets with fewer gantries due to multiple pallets being capable of being stored in each pigeonhole.
In gravity pallet flow racks and pallet push back racks, the pallets typically slide through the racks on skate wheels which are mounted on beams within the racks. In the case of gravity pallet flow racks, the pallets slide through the rack from an entry side to an exit side via the force of gravity. This is accomplished by designing the racks to be at a decline with the entry side of the rack being higher than the exit side. Pallet push back racks have only one point for insertion and removal of pallets per row of pallets in the rack. The front of each pigeonhole (row of pallets) is at a lower level than the back of the pigeonhole. Accordingly, pallets can be pushed into the pigeonholes against the force of gravity, but slide over the skate wheels on the floor of the pigeonholes back to the front of the rack as pallets are removed from the pigeonhole. Pallets are generally removed from the pigeonholes by lifting them over a stop at the front of the rack.
It is not uncommon for pallets to get misaligned and to become jammed in gravity pallet flow racks and pallet push back racks. This is undesirable because it causes down time and requires labor to clear the jammed pallet or pallets. There is accordingly a need for a low cost, low maintenance reliable system for storing pallets which efficiently utilizes warehouse space. It would also be desirable for such a system to include a means for automatic stacking and retrieval of pallets stored in the system.